In related art, there exist techniques which use biological information about the subject to determine the degree of the subject's wakefulness without imposing a burden on the subject. For example, such techniques can be applied to a vehicle to determine the degree of the driver's wakefulness, and notify the driver of impending danger.
For example, there exists related art which determines the degree of the subject's wakefulness by keeping track of changes in maximum frequency and maximum spectral density calculated by frequency analysis from the heartbeat signal of the subject (for example, International Publication Pamphlet No. WO 2008/65724). In related art, wakeful-state data, which indicates the maximum frequency and maximum spectral density in a wakeful state, that is, a state of no sleepiness serves as a reference for the degree of wakefulness. In related art, the maximum frequency and the maximum spectral density are calculated every time the heartbeat signal of the subject is acquired. Then, the degree of the subject's wakefulness is determined in real time on the basis of the amount of change of the calculated values relative to the wakeful-state data.